The present invention relates to the production of silicic acid from sodium silicates.
Silica, an inorganic material having silicon dioxide (SiO2) as a basic structural unit, is useful in a wide variety of commercial applications. Silica exists in a variety of molecular forms, which include, for example, monomers, dimers, oligomers, cyclic forms, and polymers. In addition, silica can be amorphous, crystalline, hydrated, solvated, or dry, and can exist in a variety of particulate and aggregation states.
Silica solutions exhibit polymerization behavior, resulting in the increase of Sixe2x80x94Oxe2x80x94Si bonds and decrease of Sixe2x80x94OH bonds. In an aqueous medium, amorphous silica dissolves (and/or depolymerizes), forming Si(OH)4, which undergoes polymerization to form discrete particles with internal Sixe2x80x94Oxe2x80x94Si bonds and external Sixe2x80x94OH bonds on the particle surface. Under certain conditions, the polymeric silica particles thus formed will further associate to give chains and networks comprising the individual particles.
Generally, under neutral or alkaline conditions (pH 7 or greater), the particles tend to grow in size and decrease in number, whereas under acidic conditions (pH less than 7), the particles have a greater tendency to agglomerate to form chains, and eventually three dimensional networks. If salts are present which neutralize the charge produced on the particle surface, agglomeration of particles will be more likely to occur under neutral or alkaline conditions.
The term xe2x80x9csolxe2x80x9d refers to a stable dispersion of discrete, colloid-size particles of amorphous silica in aqueous solutions. Under the proper conditions, sols do not gel or settle even after several years of storage, and may contain up to about 50% silica and particle sizes up to 300 nm, although particles larger than about 70 nm settle slowly. A sol can be formed, for example, by growing particles to a certain size in a weakly alkaline solution, or by addition of dilute acid to a solution of sodium silicate (e.g., Na2SiO3) with-rapid mixing, until the pH drops to about 8-10, followed by removal of Na+(e.g., by ion-exchange resin or electrodialysis). Silica sols, depending upon the type of silica, the particle size, and the nature of the particles, can form gels under mildly acidic to strongly acidic conditions.
The term xe2x80x9cgelxe2x80x9d refers to a coherent, rigid, continuous three-dimensional network of particles of colloidal silica. Gels can be produced by the aggregation of colloidal silica particles (typically under acidic conditions when neutralizing salts are absent) to form a three-dimensional gel microstructure. Whether a gel will form under a particular set of conditions, however, can depend on the silica properties, such as, for example, particle size and the nature of the particle surface. The term xe2x80x9chydrogelxe2x80x9d refers to a gel in which the pores (spaces within the gel microstructure) are filled with water. Similarly, the term xe2x80x9calcogelxe2x80x9d refers to a gel in which the pores are filled with an alcohol. When a gel is dried (liquid removed from the pores) by means in which the coherent gel microstructure collapses (e.g., by solvent evaporation), a relatively high density collapsed powder, or xe2x80x9cxerogelxe2x80x9d, is formed. In contrast, when a gel is dried by means in which the gel microstructure is preserved (e.g., supercritical drying as described in U.S. Pat. No. 3,652,214), a low density xe2x80x9caerogelxe2x80x9d is formed. Silica aerogels have very unusual and highly desirable properties such as, for example, optical transparency, extremely low density, and unprecedented low thermal conductivity. See Herrmann et al., Journal of Non-Crystalline Solids, 186, 380-387 (1995). Silica aerogels are useful in a wide variety of applications which include, for example, thermal insulators and reinforcing fillers for elastomers. Although raw material costs are very low, economically feasible processes for producing aerogels have been pursued unsuccessfully for decades.
The commercial success of all silica products depends on the cost and the availability of silica. The most common raw materials used in the production of silica products include sodium silicate ((Na2O)xc3x97(SiO2)y), chlorosilanes (RxSiCl4xe2x88x92x), and silicon alkoxides (Si (OR)4). Among these common raw materials, sodium silicate has the lowest cost on a per-pound basis and is a commodity chemical which is available in very large quantities. Sodium silicate can be readily reacted to produce silicic acid (Si(OH)4), from which a wide range of silica microstructures, ranging from high surface area gels to colloidal particles, can be produced. The silicic acid can be subsequently processed (e.g., gelled, precipitated, etc.) by changing the temperature, pH, and/or solids content.
One of the most significant problems associated with utilizing sodium silicate in silica production is the contamination of silica with residual sodium, which is undesirable in many applications. There are several common methods for separating residual sodium from sodium silicate-derived silica. For example, the sodium silicate can be diluted to the desired solids content and reacted with acid to make silica and an aqueous salt solution. In this situation, salt is typically removed by either washing or by adding an organic solvent to precipitate salt crystals, which are removed by decanting or centrifugation. However, washing is disadvantageous in that it yields a very dilute salt stream and further results in high residual sodium concentrations (typically greater than 100 ppm). Precipitation of salt crystals by an organic solvent also has the disadvantage of relatively high residual sodium concentrations. A third approach is to feed sodium silicate into an acid ion-exchange bed which exchanges the sodium ions with protons, providing an outlet stream of silicic acid. The ion-exchange bed approach is advantageous in that it yields the lowest residual sodium concentration. Further, the ion-exchange resin can be regenerated with acid and reused. Nonetheless, there are significant disadvantages in the production of silica using current ion-exchange methods. Typically, several fixed ion exchange beds are used, and the sodium removal and resin regeneration steps are cycled sequentially between beds. This requires high capital costs for equipment such as, for example, bed vessels, piping, and controls. Resin fouling due to gellation of silica also is a major problem, particularly in gel production. Silica rapidly gels when the pH is lowered to about 6. Gellation typically occurs in the ion-exchange resin at the reaction front, where the strong acid-base neutralization occurs. However, gellation also can occur during a process upset. Any resin fouling results in significant costs in cleanup and ion-exchange resin replacement. Production of changeover waste is also a problem. When a fixed bed ion-exchange column is shut down to be regenerated, it still contains silica and silicic acid, which contaminates the waste salt stream liberated as the column is regenerated, lowering the yield of silica and complicating salt recovery and/or disposal. Further, when a freshly regenerated column is used for sodium removal, the initial gradient of silicic acid generated on startup creates variations in the product composition, causing problems with product quality.
In view of the foregoing problems, there exists a need for an improved process for the conversion of silica from sodium silicate. The present invention provides such a process. These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.
The present invention provides a continuous process for the conversion of sodium silicate to silicic acid, wherein a moving bed of a protonated ion exchange resin, which exchanges a sodium ion in sodium silicate with a proton, is contacted with an inlet stream of sodium silicate to provide an outlet stream of silicic acid. The outlet stream of silicic acid produced thereby can be processed into a variety of silica products. When the proton exchange occurs, forming silicic acid, the outlet moving bed of spent resin becomes enriched in sodium ions. The spent sodium-enriched ion-exchange resin is continuously regenerated into protonated ion-exchange resin by contacting the spent resin with an inlet stream of acid of sufficient strength to exchange the sodium ions in the spent resin with a proton. The moving bed of regenerated protonated ion-exchange resin is continuously recycled back into the sodium silicate stream for further production of silicic acid. The sodium-enriched outlet stream produced from regeneration of the ion-exchange resin can be processed as waste or for sodium recovery.